1. Field of the Invention
The present invention generally relates to a thermal printer, and more specifically, to a thermal printer attached to a POS (Point Of Sales) device.
A thermal printer used in a POS device should be small in size and easy to set a recording sheet. This also applies to a thermal printer incorporated into a portable device.
2. Description of the Related Art
FIG. 1 illustrates an example of a conventional thermal printer. The thermal printer 10 comprises a thermal head 11 and a platen 12. The thermal printer 10 is incorporated into a sheet holder 13. The thermal head 11 is separated from the platen 12 by a cam mechanism so as to form a space between the thermal head 11 and the platen 12. A recording sheet 15 pulled out from a roll 14 is set between the thermal head 11 and the platen 12.
FIG. 2 illustrates another example of a conventional thermal printer. The thermal printer 20 is formed integrally with a sheet holder device 21. The sheet holder device 21 comprises a lower box-like member 22 and an open-close upper cover 23. The upper cover 23 is opened to set a roll 26 into the sheet holder device 21. The thermal printer 20 comprises a thermal head 24 fixed onto the inner surface of the lower box-like member 22 and a platen 25 attached to the edge of the upper cover 23. When the upper cover 23 is closed, the platen 25 is in contact with the thermal head 24. When the upper cover 23 is opened, the platen 25 is separated from the thermal head 24.
To set a recording sheet, the upper cover 23 is opened, the roll 26 is set, a recording sheet 27 pulled out from the roll 26 is pulled over the front side of the thermal head 24, and the upper cover 23 is then closed.
In the thermal printer 10 of FIG. 1, the thermal head 11 is separated from the platen 12. Due to a head pressing plate spring provided to the thermal head 11, the thermal head 11 can move only a limited distance. As a result, it is difficult to form a side sheet passage between the thermal head 11 and the platen 12. Accordingly, setting a recording sheet 15 between the thermal head 11 and the platen 12 is difficult.
Furthermore, moving the thermal head 11 might result in a deviation of the position of the thermal head 11 when it is returned. Such a positional deviation of the thermal head 11 might cause uneven printing.
To avoid deformation of the rubber platen 12, the thermal head 11 is kept separate from the platen 12 at the time of shipment of the thermal printer 10. However, moving the thermal head 11 adds to the force of the head pressing plate spring. If such a condition is maintained for a long period of time, the increased spring force deforms the main body of the thermal printer 10.
In the thermal printer 20 of FIG. 2, when the upper cover 23 is opened, the platen 25 moves. A side space is formed between the platen 25 and the thermal head 24, so that a recording sheet 27 is easy to set to this printer. However, when the platen 25 is brought back into contact with the thermal head 24, the platen 25 is substantially moved in the direction of the surface of the thermal head 24. Even a small fluctuation positioning causes a positional fluctuation of the platen 25 with respect to the heat generating member 24a in the thermal head 24. As s result, uneven printing is often carried out.
Furthermore, since the thermal printer 20 is integrally formed with the sheet holder device 21, it includes an extra portion for setting the roll 26. As a result, the thermal printer 20 is large in size.
The principal object of the present invention is to provide a thermal printer which is free of the above problems.
The object of the present invention is achieved by a thermal printer comprising a thermal head and a platen as a unit. The platen is detachable from the thermal head. The platen is moved from the thermal head so as to form a sheet passage for setting a recording sheet between the thermal head and the platen.
The thermal head is provided with a head pressing plate spring on its rear side, and movements of the thermal head are limited. On the other hand, the platen is not limited in movement. Compared with the thermal head, the platen can be moved a greater distance. Accordingly, a wider sheet passage can be formed by moving the platen instead of moving the thermal head. Thus, feeding a recording sheet into the sheet passage can be easier.
In the thermal printer of this invention, the direction of the platen separating from the thermal head is perpendicular to the surface of the thermal head.
The platen is vertically brought back into contact with the surface of the thermal head. The position of the returned platen might fluctuate with respect to the thermal printer, but the positional relationship between the platen and the thermal head is accurately maintained. Thus, uneven printing can be prevented even after the recording sheet setting is repeated many times.
The object of the present invention is also achieved by a thermal printer which comprises a main body, a thermal head attached to the main body, and platen. The thermal printer and the platen form a unit. The platen is attached to a sheet guide member for guiding a recording sheet between the platen and the thermal head. A sheet roll for holding a recording sheet is also included. A sheet guide member for guiding a recording sheet pulled out from a sheet roll into a space between the platen and the thermal head. The sheet guide member is attached to the main body. By separating the sheet guide member from the thermal head, a sheet passage for transporting a recording sheet between the thermal head and the platen is formed.
In this structure, the inlet path leading to the sheet passage is wider, and setting a recording sheet into the sheet passage is easier.
The object of the present invention is also achieved by a thermal printer which comprises a thermal head attached to a main body and a platen provided with bearings on both ends. The thermal head and the platen form a unit. The bearings of the platen are engaged with flanges on both sides of a sheet guide member for introducing a recording sheet pulled out from a sheet roller into a space between the platen and the thermal head. The platen and the sheet guide member form a platen module. The main body has bearing portions each provided with an opening. The platen module is attached to the main body, with the bearings being engaged with the bearing portions of the main body. By lifting up the sheet guide member of the platen module, the bearings are first rotated in the bearing portions, and the bearings then come out from the bearing portions through the openings. The platen is thus separated from the thermal head, thereby forming a sheet passage for setting a recording sheet between the thermal head and the platen.
By the simple action of lifting up the sheet guide member of the platen module, the platen can be separated from the thermal head.
When the platen is in contact with the thermal head, the bearings and the bearing portions vertically receive the force of the thermal head pressing the platen.
The bearing portions firmly hold the bearings, so that the platen can be fixed without runout.
When the platen is in contact with the thermal head, the bearings are subjected to a rotating force in a direction opposite to the direction of the bearings coming out from the bearing portions through the openings.
By subjecting the bearings to the rotation force in the direction opposite to the direction of the bearings coming out from the bearing portions, the bearings are firmly secured in the bearing portions.
The platen module has a mechanism for click-locking the platen module to the main body when the sheet guide member is lifted up.
In this structure, a recording sheet can be set by both hands, with the platen module being maintained in the lifted position.
The platen module also has a mechanism for click-locking the platen module to the main body when the sheet guide member is pushed back to its original position.
With this mechanism, the operator can surely recognize when the sheet setting is complete.
The object of the present invention is also achieved by a thermal printer which comprises a thermal head attached to a main body having grooves extending in a direction perpendicular to the surface to the thermal head, and a platen whose shaft is engaged with the grooves of the main body. The thermal head and the platen form a unit. The platen is guided along the grooves of the main body, so that the platen can be separated from and brought back into contact with the thermal head.
When the sheet setting is complete and the platen is brought back to the original position, the platen vertically approaches the surface of the thermal head. Even if the position of the returned platen fluctuates, the contact position on the thermal head is precisely maintained. Thus, uneven printing can be prevented.
The thermal head is engaged with the grooves and positioned by them. This adds to the positioning accuracy between the thermal head and the platen when. the platen is brought back into contact with the thermal head. Thus, uneven printing can be avoided.
The object of the present invention is also achieved by a thermal printer which comprises a thermal head module and a platen module. In the thermal head module, a thermal head and a motor are attached to a main body. The platen module includes a platen and a sheet guide member for guiding a recording sheet pulled out from a sheet roll into a space between the platen and the thermal head. The sheet guide member of the platen module is attached to the main body of the thermal head module.
The above thermal printer further comprises a gear module which reduces the rotation speed of the motor and then transmits the reduced rotation to the platen. The gear module is attached to a side surface of the thermal head module.
The thermal printer further comprises an auto-cutter module attached onto the upper surface of the thermal head module.
The thermal printer further comprises a journal takeup module for taking up a journal sheet. The journal takeup module is arranged next to the auto-cutter module on the upper surface of the thermal head module.
Having the components as modules, designing the printer is easier than designing a conventional printer.
The above and other objects and features of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings.